1. Field of the Invention
The present invention relates generally to drilling fluids used in rotary well-drilling operations and, more particularly, to oil-based drilling fluids which contain a continuous, nonaqueous liquid phase.
2. Background Discussion
When wells (such as oil, gas, and geothermal wells) are drilled using rotary well-drilling equipment, a drilling fluid is typically pumped downwardly through a string of hollow drill pipe to, and outwardly through, a drill bit connected at the bottom end of the drill pipe string. The drilling fluid--commonly referred to as a drilling "mud"--is circulated, by injection-pump pressure, back upwardly from the bottom of the drill pipe string, through the annular space formed between the drill pipe and the surrounding well bore and/or casing, to thereby transport drill bit cuttings and other well debris out of the borehole.
In addition, the flow of drilling mud performs such other important functions as cooling and lubricating the drill bit and the drill pipe (where the pipe contacts the bore walls), preventing the caving in of the borehole in weak formation regions, and overcoming natural formation pressure to thereby restrict entry into the borehole of unwanted fluids from penetrated formations.
The term "mud" applied to drilling fluids contrasts with the fact that drilling fluids are typically quite complex, specially-formulated fluids which play an important, if not critical, role in rotary well drilling operations, especially those in which deep oil or gas wells are being drilled. Drilling muds are, as an illustration, usually formulated to have: (i) a comparatively low viscosity at all temperatures normally encountered, from the wellhead to the bottom of the borehole, so as to minimize pumping problems and costs; (ii) a sufficient density to keep drill bit cuttings and debris in suspension in the event of a shutdown, to thereby prevent settling of the entrained cuttings and debris and plugging of the borehole; and (iii) a relatively high flash point so as to reduce the risk of fires and explosions which might otherwise be caused by use of the drilling mud.
For the most part, the liquid bases selected for drilling muds are aqueous solutions, oils, or emulsions of aqueous and oily materials. Various colloidal substances (such as clays), densifying components (such as barite), and/or special purpose additives (such as corrosion inhibitors, viscosity modifiers and emulsifiers), are usually added to the base liquid to enable the mud to meet the needs for particular drilling operations. These additives importantly function to increase the density and/or viscosity of the mud, to prevent drilling mud loss from the borehole into penetrated formations, and to help keep drill bit cuttings and debris suspended in the mud.
Although, principally due to their lower cost, aqueous solutions are most commonly used as the base liquid for drilling muds, the use of more costly oil-bases for drilling muds often offers important advantages over the use of water-base muds. For example, in addition to being more advantageous to use when drilling into subterranean formations which contain water-swellable clays (which can be damaged by water contact), oil-base drilling muds are usually more effective for lubricating drill bits and drill pipe, and are typically more stable than water-base drilling muds at the high temperatures ordinarily encountered when drilling deep wells. Because of these, and other, advantages, oil-base drilling muds--toward which the present invention is directed--are used in many important well-drilling operations.
Because of their comparatively lower cost and good availability, crude petroleum and petroleum distillate fractions--such as No. 2 diesel, jet "A," and kerosene--have typically been used in the formulation of oil-base drilling muds. Heavy, liquid petroleum residues, such as those containing a large amount of asphaltic materials, are also sometimes used to formulate oil-base drilling muds.
All such petroleum-based oil used for drilling muds typically contain relatively large concentrations of aromatics and at least substantial concentrations of n-olefins--both of which may be harmful or toxic to animal and plant life. Because of this, the continued use of such petroleum based oils in drilling muds may, in some situations and/or locations (for example, in off-shore drilling operations), no longer be economically practical due to the imposition of strict regulations for the protection of the environment.
It is, therefore, a principal objective of the present invention to provide a drilling mud having an base-oil which is not only non-toxic but which has other desirable properties such as low viscosity, low pour point and high flash point.